Subjects will be assigned to OBT based upon their past medication history and the results of the virus sensitivity testing (PSGT, ViroLogic, Inc.), and then randomized to a study arm to begin receiving OBT plus study medication at the Day 1 visit. Fusion/entry inhibitors will not be permitted as part of the optimized background therapy. All subjects will be randomized equally (1:1:1 ratio), in a double-blinded fashion, among three arms to receive OBT plus one of the following: Arm A, alternating intravenous infusions of TNX-355, 15 mg/kg and placebo weekly for the first 9 doses (through the Week 8 visit), and then intravenous infusions of TNX-355, 15 mg/kg every two weeks; Arm B, TNX-355, 10 mg/kg intravenous infusions weekly for the first 9 doses (through the Week 8 visit), and then intravenous infusions of TNX-355, 10 mg/kg every two weeks; or Arm C, weekly intravenous infusions of placebo for the first 9 doses (through the Week 8 visit), and then intravenous infusions of placebo every two weeks.